Doctors have warned he faces crucial few weeks after emergency surgery to treat brain haemorrhage.
He is still in a "serious condition" after emergency surgery to stop a bleed on his brain.
Sir Alex has been put into a medically induced coma, while in intensive care, after he suffered the brain haemorrhage, which is a type of stroke.
Today brain surgeons said the next 48 hours will be crucial with medics focusing on the United legend's life support.
But over the next few weeks, they will start to assess any long-term damage, and likelihood for disability.
Neurosurgeon Peter Hamlyn wrote in The Telegraph: "The most common form (of haemorrhage) for a man of his age with a history of cardiovascular issues...is intracerebral, in which there is bleeding within the brain tissue, causing irreparable damage to those cells.
"In Sir Alex's case, his surgeons have decided to operate almost certainly because they felt the size of the blood clot was causing damage to the remaining brain tissue.
"This is often life-saving surgery and aims to reduce any long-term disability Sir Alex might suffer with this form of stroke.
"If they survive without signs of improvement after several months, then it is unlikely that an individual will return to their former health.
"However, in the immediate aftermath there is everything to fight for and the potential for a full recovery, albeit sometimes after a long period of rehabilitation."
Mr Hamlyn said it is difficult to give a prognosis so soon after surgery, and while Sir Alex is still in a coma.
Meanwhile, GP Dr Mark Porter said in The Times: "Time is of the essence in treating any type of stroke.
"The outlook depends on the severity and site of the bleed but as many as half of patients with subarachnoid bleeds - a bleed on the surface of the brain - will not survive.
"And a similar proportion of those who do will be left with some form of long-term complication ranging from memory loss and speech difficulties, to epilepsy and paralysis."
Wife Cathy, who he wed in 1966, and the couple's sons Mark, Darren and Jason are all thought to be at his hospital bedside, after doctors put him into a medically induced coma.
What is shocking is some people online making up quotes there's one about Ronaldo claiming he didn't celebrate his goal vs Barca because of Fergie that was tweeted by Utd fan account. No such quote exists so unless this random person spoke to Ronaldo directly he's made it up for clicks.
The influence of the man can be seen by the way fans from all clubs have reacted, been a few wrong uns giving it out looking for attention but most of them have been put in their place by fans on that club rather than Utd fans.
Still doesn't seem to be anywhere online other than that initial big from the Daily Mail so hopefully it's true but I think all media need to keep their distance on it and let the family/doctors deal with it and they can decide when any updates are given.